Twinkle (software)

Twinkle is a free and open source software application for Voice over Internet Protocol (VoIP) voice communications in IP networks, such as the Internet. It is designed for GNU/Linux operating systems and uses the Qt toolkit for its graphical user interface. For call signaling it employs the Session Initiation Protocol (SIP). It also features direct IP-to-IP calls. Media streams are transmitted via the Real-time Transport Protocol (RTP) which may be encrypted with the Secure Real-time Transport Protocol (SRTP) and the ZRTP security protocols.
Since version 1.3.2 (September 2008), Twinkle supports message exchange and a buddy-list feature for presence notification, showing the online-status of predefined communications partners (provider-support needed).
Supported audio formats

G.711 A-law: 64 kbit/s payload, 8 kHz sampling rate
G.711 μ-law: 64 kbit/s payload, 8 kHz sampling rate
G.726: 16, 24, 32 or 40 kbit/s payload
...